System with meshed power and signal buses on cell array

ABSTRACT

A method and apparatus for providing a meshed power and signal bus system on an array type integrated circuit that minimizes the size of the circuit. In a departure from the art, through-holes for the mesh system are placed in the cell array, as well as the peripheral circuits. The power and signal buses of the mesh system run in both vertical and horizontal directions across the array such that all the vertical buses lie in one metal layer, and all the horizontal buses lie in another metal layer. The buses of one layer are connected to the appropriate bus(es) of the other layer using through-holes located in the array. Once connected, the buses extend to the appropriate sense amplifier drivers. The method and apparatus are facilitated by an improved subdecoder circuit implementing a hierarchical word line structure.

CROSS REFERENCE

This application is a Continuation of Ser. No. 10/945,351, filed Sep.20, 2004, which is a Continuation of Ser. No. 10/728,682, filed Dec. 5,2003, now issued U.S. Pat. No. 6,815,742, which is a Continuation ofSer. No. 10/315,307, filed Dec. 10, 2002, now issued U.S. Pat. No.6,831,317, which is a Continuation of Ser. No. 10/120,872, filed Apr.11, 2002, now issued U.S. Pat. No. 6,512,257; which is a Continuation ofSer. No. 09/909,191, filed Jul. 19, 2001, now issued U.S. Pat. No.6,396,088; which is a Continuation of Ser. No. 09/496,079, filed Feb. 1,2000, now issued Pat. No. 6,288,925; which is a Continuation of Ser. No.09/330,579, filed Jun. 11, 1999, now issued U.S. Pat. No. 6,069,813;which is a Continuation of Ser. No. 08/991,727, filed Dec. 16, 1997, nowissued U.S. Pat. No. 5,953,242; which is a Divisional of Ser. No.08/728,447, filed Oct. 10, 1996, now issued U.S. Pat. No. 6,115,279;which claims benefit of priority Provisional Ser. No. 60/005,502, filedNov. 9, 1995.

FIELD OF THE INVENTION

The invention relates generally to semiconductor circuit design and,more particularly, to a method and apparatus for interconnecting powerand signal buses in an integrated circuit.

BACKGROUND OF THE INVENTION

As semiconductor technology develops, the number of transistors includedin a single integrated circuit, or “chip” is becoming larger and thedesign rule parameters therefore are becoming smaller. These twodevelopments contribute to increased metal layer resistance and todifficulties associated with this increased resistance. Suchdifficulties include ground bounce, cross talk noise, and circuitdelays. All of these difficulties slow down chip operation and may evencorrupt data stored on the chip. Eliminating the impact of increasedmetal layer resistance is an important design challenge in mostsemiconductor designs, including designs for dynamic random accessmemory (DRAM) devices.

One solution to this problem has been the development of a meshed powerbus system for the chip, as described in Yamada, A 64-Mb DRAM withMeshed Power Line, 26 IEEE Journal of Solid-State Circuits 11 (1991). Ameshed power bus system is readily implemented in integrated circuitslike DRAMs because of their large arrays of memory cells and thepresence of distributed sense amplifier drivers. The meshed systemsupplies adequate power to the distributed sense amplifier driversbecause the system has many power buses running in both horizontal andvertical directions across the arrays.

The Yamada meshed system may be implemented using a conventionalcomplimentary metal oxide semiconductor (CMOS) technology, includingfirst, second and third metal layers, each electrically isolated fromeach other, wherein the first metal layer represents the lowest metallayer, the third metal layer represents the upper-most metal layer, andthe second metal layer lies between the first and third layers. TheYamada meshed system is constructed in the second and third metal layerand includes a positive supply voltage (VDD) mesh and a negative supplyvoltage (VSS) mesh, for the VDD power buses and the VSS power buses,respectively. Conventional designs have these meshes running over thememory array and connecting at the sense amplifiers. Connections aremade using through-holes, located in the area of the sense amplifiercircuits. However, the presence of VDD and VSS power buses in the senseamplifiers is unnecessary, since these circuits do not require eitherVDD or VSS power buses, except for well bias.

As a result, the sense amplifiers, due to their relatively small sizeand numerous associated signal and power buses, are adversely affectedby the Yamada meshed system. The Yamada meshed system overcrowds thesense amplifiers with additional power and signal buses. In addition,the metal line width required for overlapping through-holes is largerthan the minimum metal line width and therefore increases the width ofthe metal layers even further. As a result, the metal layer over thesense amplifiers becomes determinative of the size of the senseamplifier circuits. Accordingly, their size reduction must be realizedby tightening the metal width, inevitably resulting in increasedresistance and slower operation.

In addition to the Yamada meshed system, other proposals have been madefor conventional DRAM design. Recently, a hierarchical word line schemewas proposed in K. Noda et Al., a Boosted Dual Word-line Decoding Schemefor 256 Mbit DRAM's, 1992 Symp. on VLSI Circuits Dig. of Tech. Papers,pp. 112-113 (1992). The Noda scheme includes main word lines,constructed in the second metal line layer, and subword linesconstructed in a poly silicon layer. The Noda scheme describes two mainword lines (one true, one bar) for every eight subword lines, and isthereby able to relax the main word line pitch to four times that of thesubword line. However, this pitch would not support an improved meshedpower and signal bus system.

Consequently, there is a need for a meshed power and signal bus systemon an array-type integrated circuit that does not limit meshthrough-hole connections to the area of the sense amplifiers, butprovides for such connections at other locations on the array, therebyallowing for a relaxed metal width over the sense amplifiers and areduction of the overall area of the chip with lower power busresistance.

Furthermore, there is a need for a hierarchical word line scheme thatsupports an improved meshed power and signal bus system, that has a mainword line pitch greater than four times that of the subword line pitch.

SUMMARY OF THE INVENTION

The present invention, accordingly, is a method and apparatus forproviding a meshed power bus and signal bus system on an array-typeintegrated circuit that does not limit mesh through-hole connections tothe area of the sense amplifiers, but provides for these connections atother locations on the array, thereby allowing for a relaxed metal widthover the sense amplifiers, faster sense amplifier operation, and chipsize reduction. The through-holes for the mesh system are located in thecell array instead of, or in addition to, being located in the area ofthe sense amplifier circuits. This utilizes the available space forthrough-holes in the array, and allows for more efficient use of powerand signal buses in the sense amplifiers.

The invention includes an array of DRAM memory cells, arranged as aplurality of subarrays and selected by main address decoders. Eachsubarray is surrounded by a plurality of sense amplifiers circuits,subdecoder circuits, and VDD, VSS and signal buses connecting to andrunning across the subarray. The VDD buses run in both vertical andhorizontal directions across the subarray, with all the vertical buseslying in the third metal layer and all the horizontal buses lying in thesecond metal layer, thereby creating a VDD mesh. The buses in each layerare connected to each other using through-holes located in the memorycell subarray as well as on the sense amplifier area. Likewise, a VSSmesh and/or a signal mesh is created using through-holes located on thememory cell subarray. Once connected, the buses extend to theappropriate circuits, such as sense amplifier drive circuits, and themetal layer and through-hole requirement over the sense amplifiers issignificantly reduced.

The invention also includes a hierarchical word line scheme. Tofacilitate the combination of the above-mentioned meshed system and thehierarchical word line scheme, the Noda hierarchical word line schemeshould also be improved to provide a greater pitch of main word lines tosubword lines. In the improved hierarchical word line system, anintersection area, created between the sense amplifier and thesubdecoder, includes subdecoder drivers as well as sense amplifierdrivers. This combination provides high speed word line selection andhigh speed sense amplifier operation at the same time.

Once the sense amplifier size is no longer determined by the metalusage, as provided by the above-mentioned meshed system, an improvedlayout technique for the sense amplifier circuits may be necessary tomatch the fine memory cell size. This improved layout technique includesan alternating T-shaped gate region for a bit line equalization circuitand an H-shaped moat region with a metal-to-polysilicon-to-metal changestructure for a latch circuit.

A technical advantage achieved with the invention is the ability tofully utilize the low resistance design of a meshed power system withouthaving to increase the size of the peripheral circuits, for example,sense amplifiers, that are limited in size by their metal layers.

A further technical advantage achieved with the invention is that bothsignal and power buses may freely run in both horizontal and verticaldirections.

A further technical advantage achieved with the invention is that thedesign for through-holes located in the array area or on a stepdifference compensation area do not have to be made to the minimumdesign widths like the through-holes located in the peripheral area, andtherefore the yield is improved.

A further technical advantage achieved with the invention is that theimproved hierarchical word line structures are smaller and faster thanconventional hierarchical word line structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a 256 Mbit DRAM embodying features of thepresent invention.

FIG. 2 is a block diagram of two subarrays and surrounding senseamplifiers and subdecoders of the DRAM of FIG. 1.

FIG. 3 is a block diagram of one subarray, two sense amplifiers, and asubdecoder, as shown in FIG. 2, and a meshed power and signal systemrunning across the subarray.

FIG. 4 is a schematic diagram of a meshed power and signal system overthe subarray of FIG. 3.

FIG. 5 a is a cross sectional view of a memory cell of the subarray ofFIG. 3 with a through-hole connecting two metal layers used in themeshed power system of FIG. 4.

FIG. 5 b is a detailed schematic of a memory cell of the subarray ofFIG. 3.

FIGS. 6 a-6 c are layout diagrams of expanded sections of the meshedsystem of FIG. 4.

FIGS. 7 a-b are schematic diagrams of circuits included in theintersection area, sense amplifier, subdecoder and memory array of FIG.3.

FIG. 8 is a diagram of the subdecoder circuits of FIGS. 7.

FIG. 9 a is a schematic diagram of a prior art subdecoder circuitshowing the Noda hierarchical word line implementation.

FIG. 9 b is a schematic diagram of one subdecoder circuit showing ahierarchical word line implementation.

FIG. 9 c is a schematic diagram of a preferred subdecoder circuitshowing a hierarchical word line implementation of the presentinvention.

FIG. 10 a is a schematic diagram of the two sense amplifier circuits ofFIG. 7 a.

FIG. 10 b is a layout diagram of the sense amplifier circuits of FIG. 10a.

FIG. 11 a is a layout diagram of a circuit used in an equalizer sectionof a conventional sense amplifier.

FIG. 11 b is a layout diagram of a circuit used in the equalizer sectionof the sense amplifier circuit of FIG. 7 a, utilizing an alternateT-shaped gate region of the present invention.

FIG. 12 a is a layout diagram of a circuit used in the latch section ofthe sense amplifier circuit of FIG. 7 a, utilizing the H-shaped moatregion of FIG. 10 b.

FIG. 12 b is a simplified diagram of the H-shaped moat region of FIG. 12a.

FIG. 13 a is a metal layout diagram of a section of a conventional senseamplifier.

FIGS. 13 b-c are metal layout diagrams of an improved section of thesense amplifier of FIG. 7 a, implementing a noise decreasing method ofthe present invention.

FIG. 14 a is a first cross sectional view of a sense amplifier using atriple well structure.

FIG. 14 b is a second cross sectional view of the sense amplifier ofFIG. 2, using a triple well structure.

FIG. 14 c is a cross sectional view of the subdecoder of FIG. 2 using atriple well structure.

FIG. 15 a is a block diagram showing four fuses used for the senseamplifiers of FIG. 2 and two additional sense amplifiers.

FIG. 15 b is a schematic diagram showing four fuses used for the senseamplifiers of FIG. 2 and two additional sense amplifiers.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 the reference numeral 10 refers to a memory device embodyingfeatures of the present invention. The device 10 is fabricated using aconventional CMOS technology, including first, second and third metallayers and a polysilicon layer. The device 10 also utilizes metal oxidesemiconductor field effect transistors (MOSFETs), but other types oftransistors may also be used, such as bipolar, and metal insulatorsemiconductors. Furthermore, while in a preferred embodiment of theinvention, the device 10 is a 256 Mbit dynamic random access memory(DRAM), it should be understood that the present invention is notlimited to use with a 256 Mbit DRAM, but may be used in conjunction withother devices having arrays, including a programmable array logic, a 1Gbit DRAM and other memory devices.

The device 10 includes a set of array blocks of memory cells, such as anarray block 12, a group of pads 14 a-14 f, and a group of main addressdecoders 16 a-16 l, wherein decoders 16 b, 16 e, 16 h and 16 k are rowdecoders and decoders 16 a, 16 c, 16 d, 16 f, 16 g, 16 i, 16 j and 16 lare column decoders. The array block 12 is selected by signals from theaddress pads 14 a-14 d. It should be understood that while more addressand signal pads exist, they may be represented by address pads 14 a-14d, which are decoded by main address decoders 16 a-16 l. The mainaddress decoders 16 a-16 l represent a plurality of row and columndecoders. The row decoders generate signals including main-word signalsMWB and subdecoder control signals DXB, and the column decoders generatesignals such as column select signals YS. These signals are controlledby different address signals from the address pads 14 a-14 d, asdiscussed in greater detail below.

Array block 12, which is representative of the 16 Mbit array blocks, isfurther divided into 256 subarrays, two of which are shown in FIG. 2,and are respectively designated by reference numerals 18 a and 18 b.Each subarray consists of 128K of memory cells (arranged as 512 rows by256 columns).

Power is supplied to the device 10 through power pads 14 e and 14 f. Thepad 14 e is the positive supply voltage (VDD) power pad and is connectedto an external power supply (not shown). The pad 14 f is the negativesupply voltage (VSS) power pad and is connected to an external ground(also not shown).

Referring to FIG. 2, the memory cells of the subarray 18 a are selectedby signals from two groups of address subdecoders 20 a and 20 b.Likewise, the memory cells of the subarray 18 b are selected by signalsfrom two groups of address subdecoders 20 c and 20 d. The memory cellsof subarray 18 a are read by two groups of sense amplifiers 22 a and 22b. Likewise, the memory cells of subarray 18 b are read by two groups ofsense amplifiers 22 b and 22 c. The sense amplifiers 22 a-22 c intersectwith the subdecoders 20 a-20 d, at intersection areas 24 a-24 f. In thisway, intersection areas 24 a-24 f are created by the extension of senseamplifier areas 22 a-22 c and subdecoder areas 20 a-20 d.

Referring to FIG. 3, the pads 14 e and 14 f act as electrical ports tosupply power to the entire device 10 through main VDD and VSS powerbuses 28 and 26, respectively. The main VDD and VSS power buses 28 and26 supply power to the device 10 through a plurality of buses, locatedin different metal layers. The metal layers are layered onto a siliconsubstrate, in the order of: a first metal layer (M1), a second metallayer (M2), and a third metal layer (M3). Each of the metal layers M1,M2, M3 is electrically isolated from each other, but may be electricallyinterconnected at intersection points using through-holes. Each metallayer M1, M2, M3 also has associated therewith a thickness such that thethickness for M3 is greater than the thickness for M2, which is greaterthan the thickness for M1.

A first VDD bus 30, comprising a conductor constructed in the thirdmetal layer M3, extends in a vertical path across the subarray 18 a. Afirst VSS bus 32, also a conductor constructed in M3, extends in avertical path across the memory subarray 18 a, parallel with the bus 30.Similarly, a first signal bus 34 and a first column select YS bus 35,conductors constructed in M3, run vertically across the subarray 18 aparallel with power buses 30 and 32. A first subdecoder DXB bus 36, alsoa conductor constructed in M3, runs vertically across address subdecoder20 a, outside of the subarray 18 a.

A second VDD bus 37 a, a second VSS bus 37 b and a second signal bus 37c, conductors constructed in M3, run vertically across the subdecoder 20a and the intersection areas 24 a and 24 b. The second VDD bus 37 a andthe second VSS bus 37 b have a width that is less than a width of thefirst VDD bus 30 and the first VSS bus 32, respectively.

A third VDD bus 38 and a third VSS bus 40, along with a third signal bus42 and a second DXB bus 44, are also conductors similar to thosedescribed above, except that they are constructed in the second metallayer M2, and extend in parallel, horizontal paths across the memorysubarray 18 a. The third VDD bus 38 electrically connects with thesecond VDD bus 37 a within the subdecoder 20 a at their intersectionpoint 45 over the peripheral circuit area 20 a and the first VDD bus 30at their intersection point 46 within the memory subarray 18 a.Likewise, the third VSS bus 40 electrically connects with the second VSSbus 37 b at their intersection point 47 within the subdecoder 20 a andthe first VSS bus 32 at their intersection point 48 within the memorysubarray 18 a. Furthermore, the third signal bus 42 electricallyconnects with the second signal bus 37 c at their intersection point 49within the subdecoder 20 a and the first signal bus 34 at theirintersection point 50 within the memory subarray 18 a. Finally, thesecond DXB bus 44 electrically connects with the first DXB bus 36 attheir intersection point 52 in the subdecoder circuit 20 a. Each of theintersection points is achieved using through-holes, as discussed ingreater detail with reference to FIGS. 5 a-6 c.

Associated with each bus is a line width, it being understood that a buswith a larger surface area (width and thickness) provides a lowerresistance current path. The first VDD and VSS bus 30, 32 have a linewidth of 1.8 microns. The second VDD and VSS bus 37 a, 37 b have a linewidth of 0.7 microns. The third VDD and VSS bus 38, 40 have a line widthof 1.8 microns. Likewise, the through-holes have associated therewith adiameter, it being understood that a through-hole with a larger surfacearea (diameter) provides a lower resistance current path. Thethrough-holes located above the memory subarray 18 a have a diameter of0.6 microns, while the through-holes located above the subdecodercircuit 20 a have a diameter of 0.8 microns.

VDD and VSS power is supplied through the external pads 14 e and 14 fthe main power buses 28 and 26, respectively, as previously described inFIG. 3. The first VDD bus 30 is electrically connected to the main VDDpower bus 28 thereby supplying VDD power to the first VDD bus, thesecond VDD bus 37 a, and the third VDD bus 38. The first VSS bus 32 iselectrically connected to the main VSS power bus 26 thereby supplyingVSS power to the first VSS bus, the second VSS bus 37 b, and the thirdVSS bus 40. In this manner, a VDD mesh 54 is created by the VDD buses30, 37 a and 38 and a VSS mesh 56 is created by the VSS buses 32, 37 band 40. As a result, each of the foregoing meshes have power busesrunning both vertically and horizontally across the subarray 18 a, thesubdecoder 20 a and the intersection areas 24 a-24 b. Furthermore, theVDD and VSS meshes 54 and 56 significantly reduce the total power busresistance from the power pads 14 e and 14 f to the subdecoder 20 a, theintersection areas 24 a-24 b and other circuits, even when the widths ofthe VDD and VSS buses 37 a and 37 b are narrow.

A first peripheral circuit (not shown) drives electrical signals to thefirst signal bus 34 and the column decoder 16 a (FIG. 1) driveselectrical signals to the YS bus 35, which is used in sense amplifiers22 a and 22 b. Likewise, main address decoder 16 b (FIG. 1) driveselectrical signals to the second DXB bus 44, in a conventional manner.The first signal bus 34 electrically connects with the second signal bus37 c and the third signal bus 42 thereby creating a signal mesh 58across the subarray 18 a and the subdecoder 20 a. Likewise, the firstDXB bus 36 electrically connects with the second DXB bus 44 therebycreating a subdecoder mesh 60 across the subdecoder 20 a. In thismanner, the signal and subdecoder meshes 58 and 60 are able to connectthe sense amplifiers 22 a-22 b, the subdecoder 20 a, and theintersection areas 24 a-24 b in many different combinations. Althoughnot shown, there are many additional buses constructed in M2 andextending horizontally across the sense amplifier circuit areas 22 a and22 b. Some of these buses are connected to other signal buses, such asthe YS bus 35.

Referring to FIG. 4, the VDD, VSS, signal and subdecoder meshes 54, 56,58 and 60 actually represent many vertical and horizontal lines for eachmesh, thereby providing more buses for the surrounding circuits, anddecreasing the resistance of each mesh. For example, the subarray 18 ahas multiple VDD buses 38 a-38 d running in M2 and multiple VDD buses 30a-30 d running in M3, all tied to the main VDD bus 28 (FIG. 3), therebydecreasing the overall resistance of the VDD mesh 54. Likewise, thesubarray 18 a has multiple VSS buses 40 a-40 d running in M2 andmultiple VSS buses 32 a-32 d running in M3, all tied to the main VSS bus26 (FIG. 3), thereby decreasing the overall resistance of the VSS mesh56.

In addition to the VDD, VSS, signal and subdecoder meshes 54, 56, 58 and60, other buses run across the subarray 18 a. These other buses includemultiple column factor (CF) buses 61 a-61 d running vertically in M3,for inputs to the column decoders 16 a, 16 c, 16 d, 16 f, 16 g, 16 i, 16j and 16 l (FIG. 2), and multiple subdecoder buses (DXB1, DXB3, DXB5,DXB7) 44 a-44 d running horizontally in M2, for connection to thesubdecoder circuits 20 a and 20 b (FIG. 2) and to the first DXB bus 36.Furthermore, as shown in FIG. 4, power buses 30 a-30 d, 32 a-32 d, 38a-38 d, 40 a-40 d are located near an outer edge of the subarray 18 athan the signal buses 61 a-61 d, 44 a-44 d. As a result, resistance ofthe power buses is reduced, while the resistance for the signal buses,all grouped toward the interior edge of the subarray 18 a, arerelatively consistent with each other, thereby making signal propagationthrough the signal buses relatively consistent.

Referring to FIG. 5 a, the electrical connections between the busesshown in FIG. 4 are made at intersection points located above memorycells. An intersection point 48 a denotes where the VSS bus 32 b crossesthe VSS bus 40 b. An electrical connection is made between the VSS bus32 b and the VSS bus 40 b using a through-hole 62, located above amemory cell circuit 64. Referring to FIGS. 5 a-5 b, the memory cellcircuit 64 of the subarray 18 a comprises a conventional, one capacitorand one transistor type DRAM cell. For example, a capacitor 65 is formedbetween a plate 67 and a storage node 68. Likewise, a transistor 69 isformed with the source and drain connected to the storage node 68 and abit line (BL1) bus 70, respectively, and the gate connected to a firstsubword line (SW) bus 72 a, having a width 74. To avoid any couplingnoise caused by the power and signal buses, the cell structure of thepreferred embodiment is a capacitor on bit line (COB) structure. Thisstructure facilitates the sensitive nature of the BL1 bus 70 and enablesoperation without any detrimental effect by noise from the power andsignal meshes 54, 56 and 58 located over the cell, due to the shieldingaffect of the plate 64.

Although the intersection point 48 a appears to be located directly overthe memory cell circuit 64, this is not required, and is only for thebenefit of explanation. Furthermore, the through-hole 62 and VSS buses32 b and 40 b are not necessary for memory cell 64 and not all of thepower and signal buses will be connected to other buses.

Referring to FIGS. 4 and 6 a, a first section 76 gives an expanded viewof the subarray 18 a, showing more signal lines located between thebuses shown in FIG. 4. Section 76 has several signal and power buses ofvarious widths running both vertically and horizontally across it. Thesebuses include YS buses 35 a-35 d, having a width 80, the CF bus 61 a,having a width 82, and the VSS bus 32 b, having a width 84, runningvertically in M3. Likewise, MWB buses 86 a-86 d, having a width 88, theDXB1 bus 44 a, having a width 90 and the VSS bus 40 b, having a width92, run horizontally in M2. The signal buses YS 35 a-35 d, CF 61 a, MWB86 and DXB1 44 a run directly to their corresponding circuits, andtherefore do not require a through-hole on the subarray 18 a to changedirections. Only the VSS buses 32 b and 40 b have a through-hole 62 toelectrically connect them. With this arrangement, the width of each bus,80, 82, 84, 88, 90 and 92, is optimized for speed and power resistanceeffect. For example the widths 84 and 92 of the VSS buses 32 b and 40 b,the width 82 of the CF bus 61 a, and the width 90 of the DXB1 bus 44 aare wider than the widths 80 and 88 for high speed and low powerresistance, and to accommodate the through-hole 62. Meanwhile, the width80 of the YS buses 35 and the width 88 of the MWB buses 86, are madenarrower than the widths 82, 84, 90, 92 to conserve metal space.

Likewise, referring to FIGS. 6 b and 6 c, sections 94 and 96 are shown,having two and no through-holes, respectively. As a result, two YS busesand one CF bus ( or two YS buses and one power bus) are created withevery four sense amplifier circuits, while still meeting acceptable M3width and space requirements. Likewise, two MWB buses and one DXB bus(or two MWB buses and one power bus) are placed with every sixteensub-word-line SW buses, while still meeting acceptable M2 width andspace requirements. In addition, the widths of all the power and signalbuses may be optimized to accommodate the multiple buses used by eachmesh for reducing effective resistance and for achieving high speed,keeping the essential advantage of high yield by having the relaxedmetal pitch of hierarchical word-line configuration.

Referring again to FIG. 3, in addition to the power and signal meshes54, 56, and 58 being constructed over the subarray 18 a, they arepartially constructed over the subdecoder 20 a, along with thesubdecoder mesh 60. Other circuits are modified to accommodate the metalspace needed by the power and signal meshes 54, 56, 58 and 60,. Themodified circuits are included in the sense amplifiers, the subdecodersand the intersection areas, as described below.

FIGS. 7 a and 7 b illustrate the subarray 18 a comprising 32representative memory cells including the memory cell 64 of FIGS. 5 a-b.Furthermore, the subarray 18 a is shown in relation to the intersectionarea 24 a, the subdecoder 20 a, and the sense amplifier 22 a of FIG. 2.

In the preferred embodiment, the sense amplifier 22 a includes 128 senseamplifier circuits, such as sense amplifier circuits 98 a and 98 b. Bothof the sense amplifier circuits 98 a-98 b are connected to a senseamplifier driver 100 a, which is located in the intersection area 24 a.The sense amplifier circuit 98 a is connected to a column of memorycells 102 a, through the BL1 bus 70 (FIG. 5 a) and a bit line (BL1B) bus104 a, which are both constructed in M1, and run vertically across thearray 18 a. Likewise, the sense amplifier circuit 98 b is connected to acolumn of memory cells 102 b, through a bit line (BL2) bus 104 b and abit line (BL2B) bus 104 c, which are also constructed in M1, and runvertically across the array 18 a. Sense amplifier circuits 98 a-98 b arediscussed in greater detail with reference to FIGS. 10 a-10 b, below.

In addition to the sense amplifier driver 100 a, the intersection area24 a includes a plurality of circuits (excluding sense amplifier driver100 a and subdecoder drivers 110 a-110 d) which are referenced generallyby the numeral 100 b. These circuits 100 a-100 b are designed to employthe advantages of the low resistance of the VDD, VSS and signal meshes54, 56 and 58, as supplied by the buses 37 a-37 c.

The subdecoder 20 a includes 256 subdecoder circuits, representedgenerally be subdecoder circuit 106 a-106 d. The subdecoder circuit 106a illustrates a hierarchical word line structure utilized in each of theremaining subdecoder circuits. The subdecoder circuit 106 a is connectedto the DXB7 bus 44 d and the MWB bus 86 a, which is routed to the foursubdecoder circuits 106 a-106 d through a connector bus 108, constructedin M1. The subdecoder circuit 106 a is also connected to a firstsubdecoder driver 110 a, located in the intersection area 24 a, alongwith the sense amplifier driver 100. Likewise subdecoder circuits 106b-106 d are connected to subdecoder drivers 110 b-110 d, located in theintersection areas. The subdecoder 20 a is discussed with more detailbelow.

Referring to FIG. 8, two subdecoder drivers 110 a-110 d are located inintersection area 24 a, while the other two subdecoder drivers 110 b-110c are located in the intersection area 24 b. The subdecoder driver 110 acomprises an inverter, which converts the DXB7 bus 44 d, to an invertedsubdecoder (DX7) bus 114 d. Likewise, the subdecoder drivers 110 b-dconvert the DXB1 44 a, DXB3 44 b and DXB5 44 c to inverted subdecoderbuses DX1 114 a, DX3 114 b and DX5 114 c. In the preferred embodiment,each of the subdecoder drivers 110 a-110 d drive 64 subdecoder circuits,thereby driving all 256 of the subdecoder 20 a. Being located in theintersection areas 24 a-24 b, the subdecoder drivers 110 a-110 d aremade of significant size, and are supplied an internally generatedboosted voltage (VPP) so that the buses DX1 114 a, DX3 114 b, DX5 114 cand DX7 114 d can be driven to VPP.

The subdecoder circuits 106 a et seq. employ a hierarchical word linestructure. As discussed earlier, the subdecoder circuits formed in thesubdecoder area 20 a and 20 b are used to select certain memory cells inthe subarray 18 a. This is accomplished by utilizing a plurality ofsubword lines, such as the line 72 a, constructed in the polysilicon(FG) layer (FIG. 5 a). The MWB bus 86 a drives four subdecoder circuits106 a-106 d of subdecoder are 20 a, which each drive a SW bus 72 a-72 d,extending into the subarray 18 a. Likewise, the MWB bus 86 a drives fouradditional subdecoder circuits 106 e-106 h of subdecoder area 20 b,which each drive a SW bus 72 e-72 h, extending into the subarray 18 a.

Referring to FIGS. 9 a-9 b, a conventional subdecoder circuit 116 and analternative subdecoder circuit 118 implement a hierarchical word linestructure. The structures are hierarchical due to the placement of mainword line buses, constructed in M2, over a subword line buses,constructed in FG. However, the subdecoder circuits 116, 118 do notfacilitate the meshed system of the present invention.

Referring to FIG. 9 a, the conventional subdecoder circuit 116, as usedin the Noda hierarchical word line structure scheme, consists of threen-type metal oxide semiconductor (NMOS) transistors and produces an SWoutput. However, the subdecoder circuit 116 requires a non-inverted wordline (MW) bus, which must also run across the array (not shown) alongwith a MWB bus. This effectively doubles the number of main word linesrunning in M2 across the array. As a result, two main word lines areused to drive eight subword lines, thereby creating a pitch of 4 subwordlines to every main word line. This pitch, however, does not allow theextra metal space needed for the meshed system of the present invention.

Referring to FIG. 9 b, the subdecoder circuit 118 consists of two NMOStransistors and two p-type metal oxide semiconductor (PMOS) transistors.The subdecoder driver does not require a non-inverted word line bus (MW)as in FIG. 9 a. As a result, one main word line is used to drive eightsubword lines, thereby creating a pitch of 8 subword lines to every mainword line. But, since the subdecoder circuit consists of fourtransistors, it thereby consumes a lot of space, and to speed thecircuit up, some of the transistors must be made very large.

Referring to FIG. 9 c, the subdecoder circuit 106 a of the preferredembodiment comprises the advantages of the above two subdecoder drivers.The subdecoder circuit 106 a uses the MWB bus 86 a, the DXB7 bus 44 d,and the DX7 bus 114 d to produce the subword line SW bus 72 a, therebyallowing the subdecoder circuit 106 a to be constructed with only threetransistors 120 a-120 c. Since the DX7 bus 114 d runs only in thesubdecoder 20 a, and does not have to run horizontally across the array,the main word line pitch across the subarray 18 a remains at eightsubword lines for every main word line. As a result, there is sufficientmetal space for the power, signal and subdecoder meshes 54, 56, 58 and60, and the DXB bus 44 (FIG. 3) of the present invention.

In operation, the signals on the MWB bus 86 a and DXB7 bus 44 d,designated as MWB and DXB7, are negative logic signals, i.e. they arehigh in the standby mode, low in an enable mode. When the signals MWBand DXB7 are both low, an output signal on the subword line SW bus 72 ais driven to a selective high level. When only one of the signals MWB orDXB7 is high, the output signal on the subword line SW bus 72 a isdriven to a non-selective low level. In the standby or precharge mode,i.e., when all of the MWB and DXB signals are high, all subword lines SWare set to low.

An advantage of the subdecoder circuit 106 a is that a subthresholdcurrent in the row decoders and DXB drivers is primarily determined byNMOS transistors 120 a, 120 b. As a result, a low standby current isachieved during standby or precharge mode. This is because a gate withfor the NMOS transistors 120 a, 120 b can be narrower than that of PMOStransistors, and NMOS transistor cutoff-transition characteristics aresharper than that of PMOS transistors.

Other advantages of the subdecoder circuit 106 a are that the subdecodercircuit 106 a provides extra metal space for the power, signal andsubdecoder meshes 54, 56, 58 and 60, and the subdecoder circuit 106 aimproves in speed performance. The speed of the subdecoder circuit 106 ais directly proportional to the ability of the DX7 bus 114 d totransition from low to high. Since the DX7 bus 114 a is driven by thesubdecoder driver 110 a, and since the subdecoder driver is located inthe non-crowded intersection area 24 a, it can be made of sufficientsize. Furthermore, the DX7 bus 114 a is constructed in M3, which has thelowest resistance of the three metal layers. Thus, the DX7 bus 114 aproduces a sharp rising wave form, thereby achieving high speedactivation of the SW bus 72 a. In the preferred embodiment, a gate width(not shown) of the NMOS transistor 120 b of is narrower than that of agate width (also not shown) of the NMOS transistor 120 a, therebyimproving speed and layout area optimization. For example, in thepreferred embodiment, the gate widths of transistors 120 a and 102 b are2.2 microns and 1 micron, respectively. The narrow gate width oftransistor 120 b contributes to smaller load capacitance and faster falltimes for signals on the DXB bus 44 d. As a result, the DX bus 114 dachieves faster rise times. In addition, the gate width of 120 a is setto the sufficient value for falling speed of the subword line SW.

Referring to FIG. 10 a, the sense amplifier circuit 98 a comprises alatch section 122 a and an equalizer section 124 a. The latch section122 a comprises two NMOS transistors 126 a-126 b, connected between thebit line buses 70 and 104 a and a first latch bus 128. The latch section122 a also comprises two PMOS transistors 130 a-130 b connected betweenthe bit line buses 70 and 104 a and a second latch bus 132. All fourtransistors 126 a, 126 b, 130 a, 130 b are cross-coupled in aconventional latching manner for storing signals from the bit line buses70 and 104 a.

The equalizer section 124 a includes three NMOS transistors 134 a-134 cfor equalizing the BL1 bus 70 and the BL1B bus 104 a during the standbyor pre-charge modes. The three transistors 134 a-134 c are controlled byan equalization bus 136.

In a similar manner, the sense amplifier circuit 98 b comprises a latchsection 122 b and an equalizer section 124 b connected to the bit linebuses 104 b-104 c. The latch section 122 b and the equalizer section 124b are also connected to the two latch buses 128, 132 and theequalization bus 136, respectively.

Referring to FIG. 10 b, a further reduction in the size of the senseamplifier 22 a is achieved by other layout improvements. The equalizersections 124 a and 124 b are constructed in shapes of alternating “T's”as discussed in greater detail below with reference to FIG. 11 a. Thelatch sections 122 a and 122 b are constructed utilizing “H” shaped moatregions, as discussed in greater detail below with reference to FIG. 12a-b.

Referring to FIGS. 11 a-11 b, to reduce the size constraints of theequalizer section 124 a caused by the transistors 134 a-134 c, aT-shaped gate region 138 a (FIG. 11 a) is utilized. The equalizer signalbus 136 creates a gate for each of the transistors 134 a-134 c. In asimilar manner, the equalizer section 124 b utilizes an invertedT-shaped gate region 138 b. As a result, the gate regions 138 a, 138 bcan be compacted together, while still maintaining a required moatisolation distance 137 between the gate regions 138 a, 138 b. In sodoing, a width 140 of the two gate regions is smaller than aconventional width 142 of two square gate regions 144 a and 144 b, asshown in FIG. 11 b, and a small sense amplifier circuit 22 a correspondsto the small memory cell circuit 64 (FIG. 5 a).

Referring to FIG. 12 a, the sense amplifier 22 a also comprises anH-shaped moat 146. The BL1 bus 70, constructed in M1, must cross theBL1B bus 104 a, also constructed in M1, at the H-shaped moat 146 withoutelectrically intersecting. Furthermore, the BL1 bus 70 must drive atransistor gate 148 a and the BL1B bus 104 a must drive a transistorgate 148 b. At a crossing point 150, the BL1B bus 104 a is connected tothe transistor gate 148 b, constructed in FG, which runs under the metallayers. The gate 148 b not only serves to allow the BL1B bus 104 a tocross the BL1 bus 70, but it is the gate for the transistor 130 b. Aftercrossing the BL1 bus 70, the gate 148 b is reconnected to a connectingbus 152, also constructed in M1, thereby electrically connecting theBL1B bus 104 a to the connecting bus 152. Similarly, the BL2 bus 104 band the BL2B 104 c bus also cross in the H-shaped moat 146.

Referring to FIG. 12 b, these connections create an M1 to FG to M1change and construct the two PMOS transistors 130 a-130 b. Not only doesthis change provide a size reduction, it does so without using anadditional metal layer.

Furthermore, the H-shaped moat 146 solves another problem associatedwith the meshed system, that is, noise on the bit line buses 70 and 104a-104 c. Noise at the sense amplifiers 22 a-22 c is often caused bysignal buses constructed in M3 overlapping the bit line buses 70 and 104a-c constructed in M1. Since the bit line buses 70 and 104 a do acrossing pattern, any noise or capacitive coupling induced from signalbuses constructed in M3, such as the CF bus or the YS bus, will be thesame for both the BL1 bus 70 and the BL1B bus 104 a, thereby effectivelycanceling the effect of noise. Likewise, any noise will be the same forthe BL2 bus 104 b and the BL2B bus 104 c.

Referring to FIG. 13 a, addition noise protection from signal busesconstructed in M3 overlapping the bit line buses 70 and 104 a-cconstructed in M1 can be reduced through M2 shielding. For example, inconventional prior art designs having first and second buses 154 a-154 bconstructed in M1 and running in a vertical direction, and having athird bus 154 c constructed in M3 which also running in a verticaldirection, noise is aggravated. Noise is induced from the third bus 154c to the first and second buses 154 a and 154 b, since they overlap andrun in the same direction, allowing the noise to be strengthened by thelarge area of overlap. This conventional design can be a problem,especially when the buses 154 a, 154 b are particularly sensitive tonoise, such as the bit line buses 70 and 104 a of the present invention.Furthermore, in the conventional design, a group of other buses 156a-156 d constructed in M2 and running in a horizontal direction havelittle to no shielding effect, as shown.

Referring to FIGS. 13 b-13 c, the preferred embodiment reduces the noisebetween buses running in the same direction by improving the shieldingeffect of the M2 buses. In the preferred embodiment, the BL1 bus 70 andthe BL1B bus 104 a are constructed in M1 and run in the verticaldirection. Furthermore, the CF bus 61 a is constructed in M3 and runs inthe vertical direction, just above the two bit line buses 70 and 104 a.Located between the CF bus 61 a and the bit line buses 70 and 104 a arefour buses 158 a-158 d constructed in M2 and running in the horizontaldirections.

Referring to FIG. 13 b, one technique for reducing noise is used in asituation where the M2 buses 158 a and 158 d are noisy, active lines,such as parts of the sense amplifiers, and the M2 buses 158 b-c areinactive, quiet buses, such as a power supply bus, a first technique isused. Instead of having some of the M2 buses 158 a-158 d only extendingacross one of the bit line buses 70 and 104 a, as shown in FIG. 13 a,the M2 buses 158 b-158 c now extend over both bit line buses. In thismanner, the M2 buses 158 a-158 d provide more of a shielding affect fromany noise from the CF bus 61 a.

Referring to FIG. 13 c, in a situation where two of the M2 buses 158 aand 158 d are inactive, quiet buses, such as a power supply bus, and theother two of the M2 buses 158 b, 158 c are active, noisy buses, a secondtechnique is used. In this case, the bit line buses 70 and 104 a arebetter shielded from the noise of the CF bus 61 a by the quiet M2 buses158 a, 158 d. Therefore, the quiet M2 buses 158 a, 158 d are drawn aslarge as possible, thereby maximizing their shielding affect.

Referring to FIG. 14 a, the well structure of the sense amplifier canalso be size determinative, especially in a situation like the preferredinvention where power and signal meshes are utilized. In a first design,a triple well structure 160 comprising a p well (PW) 162 a, a deep well(DW) 164 a and a p-substrate (P-Sub) 166 is used for noise protectionfrom a sense amplifier circuit 170 to a subarray 168 a. Likewise, thetriple well structure 160 comprises a p well (PW) 162 b, a deep n-typewell (DW) 164 b and the P-Sub 166 for noise protection from a senseamplifier circuit 170 to a subarray 168 b. Although the wells 162 a, 162b, 164 a, 164 b and substrate 166 may have various bias arrangements,one such arrangement provides: TABLE 1 Well Bias Name Bias Voltage PWover DW VBBA 167a −1 V NW over DW VPP 167b 4.0 V DW VPP 167b 4.0 V P-SubVBB 167c 0 V PW (not over DW) VBB 167c 0 V NW (not over DW) VDD 167d 3.3VIt is noted that well biasing is well known in the art, and anydescriptions of bias voltage are merely illustrative, and should not belimited to such in any manner.

The subarrays 168 a and 168 b are isolated from the noisy effects of thesense amplifiers 170 by two isolation n wells (NWs) 172 a and 172 b,respectively. The NWs 172 a, 172 b create separation transistors forsharing one sense amplifier between memory cell arrays located on eitherside. A negative bias voltage that is suitable for device isolation issupplied to the P-wells 162 s and 162 b, where the above describedseparation transistors and the memory cell transistors are both located.The NWs 172 a, 172 b are biased to VPP 167 b for electrical isolation.Furthermore, the NW's 172 a, 172 b are located above the DWs 164 a, 164b, respectively, and thereby bias the DWs to VPP. The sense amplifiercircuit 170 has an additional NW 174, which is biased to VDD 167 d toprovide faster operation of a p-type transistor 176. The advantage forDWs 164 a, 164 b being biased to VPP is that the subdecoders are CMOScircuits operating at the VPP voltage level (FIGS. 7 a, 7 b, 14 c). Onthe other hand, because PMOS transistors of the sense amplifier circuit170 operate at or below the VDD voltage level, the VDD voltage level issuitable as a bias voltage for the NW 174, instead of the VPP voltagelevel. The sense amplifier 170 also has two PWs 178 a, 178 b, biased toVBB 167 c through the P-sub 166. The PW 178 a supports a transistor 180a and the PW 178 b supports transistors 180 b, 180 c.

Referring to FIG. 14 b, the preferred embodiment is able to shrink thewell structure of the sense amplifier 24 b, as compared to FIG. 14 a.The preferred embodiment utilizes a triple well structure 182 comprisinga PW 184 a, a DW 186 a, and a P-Sub 188, for subarray 18 a, and a PW 184b, a DW 186 b, and the P-Sub 188, for subarray 18 b. The subarrays 18a-18 b are thereby protected from the sense amplifier circuit 22 b. Thetriple well structure 182 also uses well-biasing similar to theillustrative biases described in Table 1. It is noted, however, thatwell biasing is well known in the art, and any descriptions of biasvoltage are merely illustrative, and should not be limited to such inany manner.

The subarrays 18 a-18 b are isolated from the noisy effects of the senseamplifiers 24 b by two isolation NWs 190 a, 190 b, respectively. Theisolation NWs 190 a, 190 b are biased to VPP 167 b for isolation.Furthermore, the isolation NWs 190 a, 190 b are located above the DWs186 a-186 b, respectively, and thereby bias the DWs. The preferredembodiment differs from the conventional system of FIG. 14 a in that theisolation NW 190 a also supports the transistor 130 d, which correspondswith the transistor 176 of FIG. 14 a. As a result, the transistor 130 dwill operate slower than the transistor 176 of FIG. 14 a. However, thespeed of the transistor 130 d is not critical to the overall timing ofthe sense amplifier circuit 90 a. Therefore, although the PMOStransistor 130 d is using a VPP biased well, there is no overall speeddegradation.

There is a size advantage, however, to the isolation NW 190 a over theconventional technique described in FIG. 14 a. Instead of having the NW172 a for the sole purpose of isolation, and the second NW 174 for thetransistor 176 (FIG. 14 a), the two are combined in the NW 190 a of thepreferred embodiment, thereby shrinking the space of the sense amplifier24 b. Furthermore, a single PW 192 can be used to support thetransistors 134 a-134 c.

Referring to FIG. 14 c, a triple well structure 193 is implemented forthe subdecoder 20 a. The P-Sub 188 and the DW 186 a extend throughoutthe subarray 18 a (FIG. 14 b), across the subdecoder 20 a, and into asubarray 196. The PW 184 a is separated from a PW 198 by an NW 200,which is biased to VPP 167 b for isolation. By biasing the NW 200 at VPP167 b, the SW bus 72 a can operate at VPP.

Referring to FIGS. 15 a and 15 b, the sense amplifier 22 a includes fourfuses 202 a-202 d used for a column redundancy scheme. The two fuses 202b and 202 d are used to disable sense amplifier circuits 98 a-98 b, andthe two fuses 202 a and 202 c are used to disable sense amplifiercircuits 204 a-204 b. Column redundancy is well known to those skilledin the art; however, conventional designs result in a dramatic areapenalty in the sense amplifier design due to the fuse placement.Accordingly, in the preferred embodiment, the fuses 202 a-202 d arelined in parallel with the bit line buses 70 and 104 a, even for thefuses corresponding to sense amplifiers located in a different area. Inthis way, the vertical running CF bus 61 a and the YS buses 35 c-35 dneed to be offset for only one group of fuses, thereby providing themaximum space for the power and signal meshes 54, 56, 58 and 60.

Although the illustrative embodiment of the present invention has beenshown and described, a latitude of modification, change and substitutionis intended in the foregoing disclosure, and in certain instances, somefeatures of the invention will be employed without a corresponding useof other features. For example, the horizontal and vertical directionswere included to make the preferred embodiment simpler to describe, butare not intended to limit the present invention. Accordingly, it isappropriate that the appended claims be construed broadly and in amanner consistent with the scope of the invention.

1. A semiconductor memory device comprising: a plurality of memoryarrays including a plurality of memory cells which are provided atintersections of a plurality of word lines and a plurality of bit lines;a plurality of first voltage lines extending in a first direction; aplurality of second voltage lines extending in a second direction; and aplurality of contacts each coupled between one of the plurality of firstvoltage lines and one of the plurality of second voltage lines, whereinthe plurality of contacts are provided above the plurality of memoryarrays.
 2. A semiconductor memory device according to claim 1, whereineach of the plurality of memory cells is a dynamic memory cells.
 3. Asemiconductor memory device according to claim 2, wherein each of theplurality of memory cells has a transistor and a capacitor, and whereineach of the plurality of contacts are provided above the capacitorincluded in corresponding one of the plurality of memory cells.